The present invention relates in general to method and apparatus for determining the alcohol content of test person's breath and, in particular, to a new useful method and apparatus for determining such alcohol content by sampling a test person's breath only after it has been blown into a sample line and attained a preselected pressure for a preselected time period.
According to present understanding of the mechanism by which the alcohol of the body is transferred to the breathing air, a reliable breath alcohol test is only possible when a volume corresponding to the clearance or shuttle volume of the test person's breath which does not take part in gas exchange with and in the lungs, has already been exhaled and after a waiting period until the so-called "deep pulmonary air" (alveolar air) is present and the alcohol concentration has assumed a saturation value.
This concept is contradicted by more recent results which show that the breathing air is also equilibrated with the blood alcohol in the mucous membranes of the mouth, in the pharyingeal cavity and in the upper respirator tracts if it is given enough time. This air thus can be used for the determination of the blood alcohol content just like the deep pulmonary air is used.
The known measuring methods and arrangements for the determination of breath alcohol concentrations are based on the present concept.
A known arrangement for the determination of alcohol concentration, measures the alcohol in the breathing air at a time determined by a time control. This time is determined by the expiration of a given time interval starting within the exhalation period. The throughput of the breathing air must not drop, within this time interval below, a given minimum throughput, and must only flow in an exhaling direction. When these two conditions are not satisfied, an error detector determines the invalidity of the measurement. The given time interval ensures that the test person has already exhaled air from the oral cavity and from the trachea at the time of measurement, and that the measuring instrument then measures the alcohol concentration of the breathing air from the alveoli of the lungs. The expiration of the time interval is determined by the time at which a minimum breathing air volume of preferably at least 80% of the total breathing air volume has been exhaled. An integrator can integrate the breathing air during inhalation and exhalation and determine therefrom the expiration of the time interval after the minimum breathing air volume. This embodiment is to be independent of the physical structure of the test person. The method however, is not immune to measuring errors, caused by an uncooperative test person. By deliberate flat inhalation, the test person can simulate a low breathing capacity. The minimum breathing air volume, which is automatically established at 80% of the total breathing air volume, for example, can then originate practically only from the oral and pharyngeal cavity. The alveolar air, which is determined for an accurate measured value, is then not fully determined (DOS No. 24 28 352).
Another known method, and also the respective arrangement, solve this problem by means of an infrared measuring instrument which constantly measures the instantaneous alcohol concentration during the sampling.
A threshold comparator determines the variation of the measured values per unit of time, which is a measure for the rate of rise of the alcohol concentration.
A measured value is only transmitted when the rate of rise drops below a given threshold value. This first condition results from the fact that the portion of the shuttle air from the oral and pharyngeal cavity diminishes constantly with a dropping rate of rise, and that only alveolar air is in the measuring channel of the arrangement when it drops below the threshold value. Another condition for the transmission of the measured value is that the velocity of flow of the exhaled air determined by a flowmeter must have been above a given value during a given period before the measured value is transmitted. This additional condition ensures the provided course of the measuring method. The measurement of the alcohol concentration is effected by an infra-red measuring instrument with a short response time connected into the breathing air current. A disadvantage is that, because of the high resolution of the measured values, as is necessary for determining the rate of rise, an elaborate infra-red measuring instrument is required. A reliable determination of the alveolar air portion is not possible with a simple, inexpensive, but slow alcoholometers (DOS No. 26 10 578).